Download - S4 T1 Suentjens Small Field Dosimetry
Small Field Dosimetry for IMRT and Radiosurgery
Jan Seuntjens, Ph.D., FAAPM, FCCPMProfessor & Director Medical Physics
McGill UniversityCanada
SEAAPM 2011Thursday April 7 1
Overview – “Small field” radiotherapy
• Background– Issues and definitions
• Measurement physics of small fields
• Measurement physics of IMRT (composite) fields
• New dosimetry of small and IMRT fields –status report
• Conclusions
SEAAPM 2011Thursday April 7 2
Acknowledgments – small field committees
• IAEA committee– Palmans (Chair)– Andreo– Huq– Mackie – Ulrich– Kilby– Izewska– Capote– Alfonso– Seuntjens
• AAPM Committees– TG‐178 (Goetsch et al)– TG‐155 (Das et al)– WGDPCB
• ICRU Report Committee
Thursday April 7 SEAAPM 2011 3
Background
• Technological improvements in conventional Linacs have improved mechanical accuracy and stability as well as dosimetric control
• Increased availability of tertiary MLCs on conventional accelerators (Brainlab M3, NovalisTx)
• Specialized machines for IGRT using small fields or combination of small fields (GammaKnife, CyberKnife, TomoTherapy, Vero)
Has technology gotten ahead of comprehension of basic dosimetry principles?
Thursday April 7 SEAAPM 2011 4
Summary of some challenges in small‐field dosimetry
• Definition of field size?
• Difficulties in accurate measurements
• Modeling of small field dose calculations in TPSs
• Calibration protocol reference conditions cannot be achieved
Thursday April 7 SEAAPM 2011 5
IMRT versus SRS/SRT – some nomenclature
• nonstandard (a.k.a. non‐compliant) fields: radiation fields for which reference dosimetrycannot be reliably performed using the existing protocols (AAPM TG‐51 or IAEA TRS‐398)– Small fields (static)
• Reference conditions cannot be met (10 x 10 cm2 is not available)
– Composed fields (IMRT, step‐and‐shoot or dynamic)• Delivery conditions are far removed from calibration conditions
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Large differences in Output Factorsamong users/machines
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Statistics of 45 Output Factors for 6 mm and 18 mm square fields Novalis, SSD = 100 cm, depth = 5 cm, various detectors)
Statistics of 45 Output Factors for 6 mm and 18 mm square fields Novalis, SSD = 100 cm, depth = 5 cm, various detectors)
From Wolfgang Ullrich (BrainLab)
factor of 2 in dose determination!
Physics of small fields
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What constitute small‐field conditions?
• Beam‐related small‐field conditions– the existence of lateral charged particle disequilibrium
– partial geometrical shielding of the primary photon source as seen from the point of measurement
• Detector‐related small‐field condition– detector size compared to field size
Thursday April 7 SEAAPM 2011 9
What constitute small‐field conditions?
• Beam‐related small‐field conditions– the existence of lateral charged particle disequilibrium
– partial geometrical shielding of the primary photon source as seen from the point of measurement
• Detector‐related small‐field condition– detector size compared to field size
Thursday April 7 SEAAPM 2011 10
ICRU, September 2008 11
Lateral charged particle loss
broad photon field
volume volume
narrow photon field
A small field can be defined as a field with a size smaller than the “lateral range” of charged particles
is a measure of the degree of equilibrium or transient equilibrium
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Li et al. 1995 Med Phys 22: 1167‐70
Loss of lateral charged particle equilibrium
Concept of rLEE
Source occlusion
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Large field conditions Small field conditions(IPEM Report 103)
Overlapping of beam penumbras
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From Das et al. 2008 Med Phys 35: 206‐15
• definition of field size?
What constitute small field conditions?
• Beam‐related small‐field conditions– the existence of lateral charged particle disequilibrium
– partial geometrical shielding of the primary photon source as seen from the point of measurement
• Detector‐related small‐field condition– detector size compared to field size
Thursday April 7 SEAAPM 2011 15
Detector dependence of output factor
Thursday April 7 SEAAPM 2011 16From Doblado et al. 2007 Phys Med 23:58‐66
Detector issues in small field dosimetry
• Energy dependence of the response
• Perturbation effects– Central electrode
– Wall effects
– Fact that cavity is air‐filled instead of water
– Volume averaging
• Interaction of these effects with beam focal spot size
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Detector‐related small field condition
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One could claim that the GammaKnife 18 or 14 mm diameter fields are not small (quasi point source + electron equilibrium length about 6 mm)
Meltsner et al., Med Phys 36:339 (2009)
Exradin A16 inner diameter
Exradin A16 outer diameter
Measurements with small‐field detectors
Thursday April 7 SEAAPM 2011 19
Sauer & WilbertMed Phys 34, 1983‐88 (2007)IC = PTW 31010 (0.125 cm3)PiP = PTW 31006 (Pinpoint)
SES = size of equivalent square
Why do we worry about CPE or TCPE?
Thursday April 7 SEAAPM 2011 20
Valid for CPE or TCPE.Correction factor kQ well‐known for:
Q1 = Co‐60; Q2 = linac beam with field10 x 10 cm2; z = 10 cm; SSD or SAD 100 cm
Why do we worry about CPE or TCPE?
For the measurement of absorbed dose:• In regions of CPE and TCPE: SPR conversion is accurate
– Dosimetry according to TG‐51 or TRS‐398
• In regions of non‐CPE: SPR conversion is not accurate and additional, sometimes large, corrections are needed, i.e., – Dosimetry in narrow stereotactic fields
• In regions of “recomposed‐CPE” (IMRT, Tomotherapy, etc): we don’t know (since composition of CPE may be disturbed by detector, or may not be perfect)– Dosimetry of intensity modulated fields
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22Sanchez‐Doblado et al Phys. Med. Biol. 48 2081‐2099 (2003)
Small field dosimetry
0.3% effect0.3% effect
Stopping power ratio water to air
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Eklund and Ahnesjö, Phys Med Biol 53:4231 (2008)Eklund and Ahnesjö, Phys Med Biol 53:4231 (2008)
Very small effects!
24
Narrow 1.5 mm fieldRatio of dose to water to dose to air averaged over cavity volume
Off‐axis distance (mm)
Collecting electrode diameter: 1.5 mmSeparation: 1 mm
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
0 2 4 6 8
Dw/D
air
Stopping power ratio w/air
Paskalev, Seuntjens, Podgorsak (2002) AAPM Proc. Series 13, Med. Phys. Publishing, Madison, Wi, 298 – 318.
60%
Role of different perturbation factors in small fields
Thursday April 7 SEAAPM 2011 25
080915
Crop et al., Phys Med Biol 54:2951 (2009)
PP31006 and PP31016 chambers
Off‐axis behaviour of correction factors
Thursday April 7 SEAAPM 2011 26
080915
Crop et al., Phys Med Biol 54:2951 (2009)
8 mm x 8 mm field, 10 cm depth (0.6 mm, 2 mm spot sizes)
Very small effects!
Off‐axis behaviour of correction factors
Thursday April 7 SEAAPM 2011 27
080915
Crop et al., Phys Med Biol 54:2951 (2009)
8 mm x 8 mm field, 10 cm depth (0.6 mm, 2 mm spot sizes)
Very large effects!
Summary of measurement issues in small fields
• Beam dependent issues– Beam focal spot size plays a role
– Lateral disequilibrium
– How do we measure beam quality in practice?
• Detector effects– There is no ideal detector
– Volume averaging and perturbation effects
– Corrections depend on beam focal spot size
Thursday April 7 SEAAPM 2011 28
Physics of IMRT fields
Thursday April 7 SEAAPM 2011 29
Composite nonstandard fields
• Composed of multiple small fields
• Thus: same dosimetric issues as in small fields– Lateral charged particle disequilibrium
– Partial occlusion of source
– Detector response problems, volume averaging, perturbations
• How can dosimetric accuracy be affected?
Thursday April 7 SEAAPM 2011 30
Dose measurements in composite fields
Thursday April 7 SEAAPM 2011 31
=
=
0.973( ) 0.022NE
NE
xs x
=
=
0.963( ) 0.024IC10
IC10
xs x=
=
0.944( ) 0.035PP
PP
xs x
Fraser et al (2009) JACMP 10 (4): 241‐51
Why do we worry about CPE or TCPE?
For the measurement of absorbed dose:• In regions of CPE and TCPE: SPR conversion is accurate
– Dosimetry according to TG‐51 or TRS‐398
• In regions of non‐CPE: SPR conversion is not accurate and additional, sometimes large, corrections are needed, i.e., – Dosimetry in narrow stereotactic fields
• In regions of “recomposed‐CPE” (IMRT, Tomotherapy, etc): we don’t know (since composition of CPE may be disturbed by detector, or may not be perfect)– Dosimetry of intensity modulated fields
Thursday April 7 SEAAPM 2011 32
Thursday April 7 SEAAPM 2011 33
Field name CIMRT,6 MVmeasured CIMRT,6 MV
calculated DifferenceStatic #1 1.019 ± 4.0% 0.950 6.8%
Static #2 1.173 ± 6.1% 1.150 1.9%
Static #3 1.124 ± 3.2% 1.094 2.7%
Static #4 1.274 ± 6.0% 1.233 3.2%
Static #5 1.172 ± 2.5% 1.141 2.7%
Dynamic #1 1.139 ± 3.0% 1.143 -0.3%
Dynamic #2 1.169 ± 2.5% 1.161 0.7%
Dynamic #3 1.089 ± 3.9% 1.004 7.8%
Dynamic #4 1.007 ± 3.2% 1.031 -2.4%
Dynamic #5 0.920 ± 5.4% 0.885 3.8%
Dynamic #6 1.583 ± 5.9% 1.605 -1.4%
Dynamic #7 1.077 ± 5.7% 1.014 5.9%
Dynamic #8 1.079 ± 5.9% 1.005 6.9%
Bouchard &Seuntjens, Med Phys 31: 2453‐2464 (2004)
Thursday April 7 SEAAPM 2011 34
Exradin A12 Exradin A14
Gradient correction due to volume averaging is dominating
Issue with reference dosimetryconditions
Thursday April 7 SEAAPM 2011 35
Modality Most relevant static calibration
field size
S&S or dynamic capabilities?
IMRT, SRS/SRT
10 x 10 cm2 Yes
TomoTherapy 5 x 20 cm2 Yes
CyberKnife 6 cm diameter Yes
GammaKnife 16 mm / 18 mm diameter
Yes
New dosimetry of small and IMRT fields
Status Report
Thursday April 7 SEAAPM 2011 36
Several working groups and TG’s
• AAPM TG 155 – small field relative dosimetry
• AAPM TG 178 – GammaKnife dosimetry
• IAEA small field committee – liaised with AAPM WGDPCB
• ICRU report committee on prescribing and reporting of stereotactic radiation therapy
• IPEM – Report 103
• DIN – small field subcommittee
Thursday April 7 SEAAPM 2011 37
Upcoming guidelines and recommendations
• New formalism• Data for new formalism• Practical issues
– Small fields:• Beam quality• Suitable detectors• Correction factors• etc
– Composite fields IMRT:• Beam quality• Suitable detectors• Reference field criteria• Correction factors• etc
Thursday April 7 SEAAPM 2011 38
New formalism
• Two related routes for Dw in non‐conventional reference conditions, both requiring an extension of concept of reference field and modified reference conditions
• Small static field dosimetry– intermediate machine‐specific‐reference field (msr)
– Recommendations for correction factors to measured output factors
• Composite field – IMRT dosimetry– plan‐class specific reference field (pcsr)
– A pcsr field can be a 3‐D irradiated volume or a 4‐D delivery sequence.
– The pcsr should be as close as possible to a class of clinical plans of interest, and provide a uniform dose over a region exceeding thedimensions of a reference detector
Thursday April 7 SEAAPM 2011 39
Alfonso et al Med Phys 35: 5197 (2008)
Static small fields
Thursday April 7 SEAAPM 2011 40
Thursday April 7 SEAAPM 2011 41
0 0
,, , , , ,
msr refmsr msr
msr msr msr
f ff fwQ Q D wQ Q Q Q QD M N k k= msrclin
msrclin
msr
msr
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ffQQ
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fQw DD ,
,,, Ω=Machine specificreference field fmsr
Clinicalfclin
Tomotherapy5 cm x 20 cm
REFERENCE DOSIMETRY RELATIVE DOSIMETRY
GammaKnife∅ 1.6/1.8 cm
CyberKnife 6 cm
1
≡ Ionizationchamber
Broad beamreference field fref
00 ,,, QQQwD kNHypothetical
reference field fref
BrainLABmicro MLC10cmx10cm
refmsr
msr
ffQQk ,,
Radiosurgical collimators∅ 1.8 cm
refmsr
msr
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msrclinmsr
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ffQQf
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,,, ⋅=Ω
How to specify beam quality?
Thursday April 7 SEAAPM 2011 42
Sauer (2009) Med. Phys. 36: 4168
Multiple beams from BJR Suppl 25
Thursday April 7 SEAAPM 2011 43
AAPM TG‐148 (Langen et al. 2010 Med Phys 37:4817‐53): “dd(10)x[HT‐ref]”
Beam quality specifier for Tomotherapy
Data for
Thursday April 7 SEAAPM 2011 44
• correction factors are small for the larger field msr
Data for (cont’d)
Thursday April 7 SEAAPM 2011 45
• correction factors are small for the larger field msr
Sterpin et al (2011 ‐ preliminary data)
Tomotherapy msr field
Field output factors
Thursday April 7 SEAAPM 2011 46
)1()2(
)2()2(
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,
,,,
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080915 47/25
Field output factors ‐ CyberKnife: Pantelis et al. 2010 Med Phys 37:2369
0.600
0.650
0.700
0.750
0.800
0.850
0.900
0.950
1.000
1.050
0 5 10 15 20
diameter / mm
M /
M60
A16PinPointDiode 60008Diode 60012EDGEAlanineTLDEBT filmPolymer gel
0.950
1.000
1.050
1.100
1.150
1.200
1.250
1.300
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diameter / mm
(M/M
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M/M
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PinPointDiode 60008Diode 60012EDGEAlanineTLDEBT filmPolymer gel
0.950
1.000
1.050
1.100
1.150
1.200
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Getting output factor data for multiple detector types
Thursday April 7 SEAAPM 2011 48Sauer and Wilbert 2007 Med Phys 34:1983‐8
Output factors – CyberKnifecoupling of beam spot size and detector correction factors
Thursday April 7 SEAAPM 2011 49
Francescon et al 2008 Med Phys 35:504‐13
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IMRT fields
Thursday April 7 SEAAPM 2011 50
New formalism
• Two related routes for Dw in non‐conventional reference conditions, both requiring an extension of concept of reference field and modified reference conditions
• Small static field dosimetry– intermediate machine‐specific‐reference field (msr)
– Recommendations for correction factors to measured output factors
• Composite field – IMRT dosimetry– plan‐class specific reference field (pcsr)
– A pcsr field can be a 3‐D irradiated volume or a 4‐D delivery sequence.
– The pcsr should be as close as possible to a class of clinical plans of interest, and provide a uniform dose over a region exceeding thedimensions of a reference detector
Thursday April 7 SEAAPM 2011 51
Alfonso et al Med Phys 35: 5197 (2008)
Thursday April 7 SEAAPM 2011 52
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ffQQk,,
refpcsr
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,,, Ω=Broad beamreference field
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pcsr
(e.g. IMRT Linac)
refmsr
msr
ffQQk ,,
fmsr
(e.g. Tomotherapy5cm x 20cm)
REFERENCE DOSIMETRY RELATIVE DOSIMETRY
20º 60º
100º
140º180º220º
260º
300º 340º
2
≡ Ionizationchamber
pcsrclin
pcsrclinpcsr
pcsr
clin
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Q
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Thursday April 7 SEAAPM 2011 53
Dynamic IMRT H&N – Chung et al. 2010 Med. Phys. 37:2404‐13
TomoTherapy – Bailat et al. 2009Med. Phys. 36:3891‐3896
VMAT – Rosser and Bedford 2009 Phys. Med. Biol. 54:7045‐7061
0.97
0.98
0.99
1.00
1.01
1.02
1.03
pcsr field
kpc
sr,re
f
Bailat et al. 2009
Rosser and Bedford 2009
Chung et al. 2010
pcsr field correction factors
0.97
0.98
0.99
1.00
1.01
1.02
1.03
pcsr field
kpc
sr,re
f
Bailat et al. 2009
Rosser and Bedford 2009
Chung et al. 2010
k=2
Suitable IMRT calibration fields?
Thursday April 7 SEAAPM 2011 55
Chung et al 2011
What are possible criteria for suitable IMRT calibration fields?
Thursday April 7 SEAAPM 2011 56
Chung et al 2011
Conclusions
• Small and IMRT field dosimetry can be complex– There are hefty perturbation effects that can have significant impact on reference dosimetry procedures and output factors
– Comparison between different detectors provides valuable information
• In small field dosimetry– Machine‐specific reference fields defined
– Data on correction factors is being collected
Thursday April 7 SEAAPM 2011 57
Conclusions (cont’d)
• In IMRT / composite field dosimetry– Measurement uncertainties, reference detectors– Practical criteria for the reference field definition are being developed
• New documents will be coming out providing guidelines on how to better deal with these issues
Continued education is needed and can prevent complacency
Thursday April 7 SEAAPM 2011 58